Analytical potential energy surfaces for ethynyl, acetylene, and vinyl

Analytical potential energy surfaces are developed for the lowest electronic states of ethynyl (C₂H), acetylene (C₂H₂), and vinyl (C₂H₃) based on fits to accurate ab initio calculations for the respective molecular force fields, and using experimentally derived energies of formation. The surfaces are global in that the energy defects associated with all rearrangement and dissociation processes on each surface are at least approximately correct. In addition, there are no significant spurious minima and the surfaces are sufficiently smooth that trajectories may be integrated on them without difficulty. In fitting the dissociation and rearrangement barriers, we emphasized those barriers which are important in the H + C₂H₂ addition reaction. Thus, the barrier for the addition reaction is optimized to equal the current best estimate of its value, as is that for the next lowest energy process, H atom migration in C₂H₃. Barriers for higher energy steps such as H + C₂H₂ → H₂ + C₂H and H atom migration in C₂H are within 10 kcal/mol of the current best estimates. The only process that is not well described on our surface is the vinylidene-acetylene isomerization. The fitting procedure used for C₂H combines a LEPS potential with a three-body Sorbie-Murrell function to fit the C₂H force field, the isomerization barrier, and the C₂H dissociation energies. For C₂H₂, two C₂H fragment potentials are combined with an empirical methylene potential and a four-body Sorbie-Murrell function to fit the acetylene force field, the vinylidene minimum energy, and other information. The potential for C₂H₃ combines information about the acetylene fragments with an empirical methyl/H₃ potential and a five-body Sorbie-Murrell function to fit the vinyl force field, the addition and migration barriers, and various dissociation and rearrangement energies.